To meet recent demands for use on vehicles or for employing DC power supplies for large tools, small and lightweight secondary batteries capable of performing rapid charge and large-current discharge have been required. Examples of typical batteries satisfying such demands include a nonaqueous electrolyte secondary battery employing a material capable of inserting and extracting lithium ions, such as carbon, as a negative electrode active material, a material reversibly and electromechanically reacting with lithium ions, such as lithium cobaltate composite oxides, as a positive electrode active material, and an aprotic organic solvent in which lithium salt, such as LiClO4 or LiPF6, is dissolved, as an nonaqueous electrolyte.
Such a nonaqueous electrolyte secondary battery (hereinafter simply referred to as a “battery”) has a configuration in which an electrode group is housed in a battery case together with an electrolyte, and an opening part of the battery case is sealed with a sealing plate, the electrode group being formed by winding or stacking, with a separator (a porous insulating layer) interposed, a positive electrode including a positive electrode current collector on which a positive electrode active material is formed and a negative electrode including a negative electrode current collector on which a negative electrode active material is formed.
Incidentally, occurrence of an internal short circuit in the nonaqueous electrolyte secondary battery causes current to flow in the battery, resulting in a temperature rise in the battery. Factors in occurring an internal short circuit may vary. Particularly, destruction by crash of a battery immediately causes a large current flow, resulting in a rapid temperature rise in the battery.
In general, a porous insulating layer (e.g., a polyolefin layer) used as a separator has a so-called shutdown function, a function of allowing no current to flow by becoming nonporous when the temperature in the battery becomes high due to a temperature rise by an internal short circuit. However, severe heat generation may melt and shrink the porous insulating layer to expand a portion where a short circuit occurs. Hence, suppression of abnormal heat generation is difficult.
In view of this, Patent Literature 1 discloses, as a method for suppressing such abnormal heat generation, the use of a separator in a layered structure of a porous insulating layer having the conventional shutdown function and a heat-resistant porous insulating layer (e.g., a polyimide layer, an aramid layer, etc.). A separator having such a layered structure can maintain the inherent shutdown function, and can prevent expansion of a portion where a short circuit occurs through the heat-resistant porous insulating layer when the shutdown function does not work by severe heat generation, thereby suppressing abnormal heat generation.
Further, Patent Literature 2 discloses a method for suppressing abnormal heat generation, in which the resistivity of a positive electrode active material is increased to suppress the magnitude of short circuit current flowing at an internal short circuit.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-100408    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-297763    Patent Document 3: Japanese Unexamined Patent Application Publication No. 5-182692    Patent Document 4: Japanese Unexamined Patent Application Publication No. 7-105970